I'm not sure what it is, but whenever my friends are playing it, they become super unpleasant, more so than any other game.

Hahaha that sounds awfully familiar. One of my friends turned into such a grinch, and got on my nerves to such an extent during a period of DotA in college, that I basically haven't included him in any of my multiplayer-sessions ever since.. He's a nice guy normally, albeit a bit over-competitive, and man does DotA bring out that trait.... He'd start with the blame-game (or course it was always someone else's fault that his masterful plan failed), and then the silent-treatment on ventrilo to the ridiculous point of refusing to answer even when spoken directly to. But of course neither leaving the game or vent, oh no. Had to stick around to sour the mood and not get penalty points for leaving obviously.